A reagent consisting of tetrachloropalladate (II), iodate, and leuco crystal violet (4,4',4"-methylidynetris(N,N-dimethylaniline) produces crystal violet in proportion to the concentration of carbon monoxide and the time of reaction. At pH 3.1, the reaction between iodate and leuco crystal violet is kinetically inhibited, but the reduction of palladium(II) to palladium(O) by carbon monoxide followed by the reduction of iodate by palladium(O) produces hypoiodous acid which rapidly oxidizes leuco crystal violet to crystal violet. As palladium(II) is regenerated in the reaction of iodate with palladium(O), no metallic palladium appears in the solution. A further catalytic reaction between the iodate produced in the reaction of hypoiodous acid with leuco crystal violet and excess iodate is apparently of some importance in the production of crystal violet. This reaction has been adapted to the analysis of carboxyhemoglobin by use of a modified Conway microdiffusion cell in which the addition of sample and reagents, and the spectrophotometric measurement of the absorbance of the crystal violet produced, are carried out in a closed system.